An organically grown planet definition

In early 2017, the two of us, along with a few others, refreshed the debate on the definition of planet in scientific nomenclature. The International Astronomical Union’s (IAU) historic definitional vote in 2006 recognized only eight solar system planets, and this has brought new focus to some underlying issues of importance to planetary science. Specifically, this debate touches on how words acquire their meaning and shape our thinking in both science and everyday life. Accordingly, the definition of planet is about much more than whether students learn Pluto’s name in a list of planets.

Words are also how we scientists and educators communicate science to the public. In other words (so to speak), words structure our understanding of the world. This mental structure is what educational psychologists call a schema. Scientists define words as part of our scientific nomenclature with an eye toward schematic usefulness to conceptualize, organize, synthesize, and contextualize information about nature. Nomenclatures’ definitions arise organically: Scientists choose words and phrases to describe their work, and write them in peer-reviewed journals and periodicals, and speak them aloud in scientific conferences and classrooms. Precedent is a key element in forming definitions (just ask a lawyer!) that both reflect and promote a useful schema for understanding the natural world.

Conversely, scientific definitions are almost never and should never be handed down authoritarian-style from a central voting body, particularly when scientists of different disciplines have different uses for the same word. The artificial authority behind the few voted definitions in existence, such as the IAU’s planet definition, should be viewed with skepticism and even dismissal. Science functions through individual experts making conclusions and coming to consensus, rather than being instructed on what has been decided. For instance, as far as I (Kirby) know as a planetary geologist, no one has ever voted on the definition for a barchan sand dune. Yet, through usage and precedent, a definition for barchan exists based on its introduction in the scientific literature in 1881 by Alexander von Middendorf. Britannica’s useful definition for barchan sand dune is based on the word’s precedent in the literature; the definition is a “crescent-shape sand dune produced by the action of wind predominantly from one direction … with a gentle slope facing toward the wind and a steep slope, known as the slip face, facing away from the wind.” (It so happens that barchan sand dunes are all over the place on Mars!)

That definitions arise through professional and common usage are one blow against the legitimacy of the IAU’s definitional vote. Another blow arises from the fact that scientists of one discipline should not presume to define words for another. An illustration stems from considering the word metal. Astronomers use it to describe elements in stars heavier than helium. Metallurgists use the word in the more common way, yet astronomers and metallurgists don’t fight over the definition — each user community knows what they mean when they use the word metal. What would happen if the metallurgical community declared an official definition of metal and then publicly scorned astronomers for using a different definition, saying, “I wish they would just get over it”?

Just as different definitions of metal serve different communities, we, as planetary scientists, find it useful to define a planet as a substellar mass body that has never undergone nuclear fusion and has enough gravitation to be round due to hydrostatic equilibrium, regardless of its orbital parameters. This is the definition we presented at the 2017 Lunar and Planetary Science Conference. Indeed, planetary scientists already use and teach such a geophysical definition of planet to promote a useful mental schema about the round and non-round worlds we study: At least 119 peer-reviewed papers in professional, scientific journals implicitly use this definition when they refer to round worlds (including moons) as planets. The publication history for these papers spans decades, hailing from both before and after the 2006 IAU vote. This overwhelming precedent cements the geophysical definition’s legitimacy in professional planetary science.

We realize that more than 100 objects in the solar system fit this geophysical planet definition, yet this does not dilute the word’s usefulness. Rather, subcategories of planets help us form a mental schema to recognize planets’ diversity and then draw conclusions and insights based on groupings of planets with similar properties. A few useful examples of diversity among planets include terrestrial planets (Mars), giant planets (Uranus), dwarf planets (Pluto, Eris, etc.), and satellite planets (Europa). Each subcategory of planet helps us recognize similarities and differences: the domain of comparative planetology. For instance, Enceladus (an icy dwarf satellite planet) and Neptune (a giant planet) are very different types of planets in size, gravity, and orbits. Yet both are round, contain high amounts of water, and are located within our solar system’s Middle Zone. (This usage further illustrates the nonsensical claim by the IAU that dwarf planets are not planets; rather, dwarf planets are a subcategory of planets just as giant planets are.) Just as having approximately 400 billion objects that fit the definition of star in our Milky Way Galaxy does not diminish the usefulness of the word star, likewise having many planets in our solar system does not diminish the usefulness of the word planet. Similarly, stars’ size and spectral diversity between red dwarf stars and blue supergiant stars parallels planets’ diversity between small Kuiper Belt dwarf planets and giant planets.

Definitions, like numerical measurements, have uncertainty, or as scientists like to call it, an “error bar.” Many small quasi-round worlds fall into that uncertainty on the small end of the size spectrum, and deuterium-fusing large worlds (brown dwarfs) fall into the error bars on the large end. However, the geophysical definition of planet has low enough uncertainty to still be useful to us.

The geophysical definition further proves its worth when considering exoplanets orbiting other stars outside our solar system. As a thought experiment, assume our Milky Way Galaxy has a conservative 100 billion possible planetary systems anchored by at least one star. Assume a conservative 100 dwarf planets like Pluto or Eris in each system. That’s 10 trillion dwarf planets in just our galaxy. If one assumes five giant planets per planetary system, that’s only 500 billion giants in the galaxy compared to 10 trillion dwarfs. Thus, dwarfs outnumber giants 20 to 1 and are the rule rather than the exception. Re-formulating our schema of what a planet is facilitates such insights.

This new schema for planet — properly defined by expert planetary scientists — will powerfully work itself out in grade school classrooms. Rather than teaching students the names of all the planets, teachers should emphasize the types and subtypes of planets and how the solar system is naturally organized outward from the Sun, using a handful of planets as examples. This is analogous to learning the organization of the periodic table of the elements without having to memorize all or even most of the 100+ names.

Along with this teaching strategy, scientists, educators, and students should ignore illegitimate scientific definitions that arise via voting, such as the IAU’s planet definition. Instead, they should adopt definitions that arise naturally through usage by experts in the field, which reflect and promote a useful mental schema about the natural world and a more accurate picture of how science operates.

Other scientists may find a different definition useful, such as one more concerned with orbits and gravitational effects on smaller worlds, as proposed by the IAU. However, such scientists should not look to the IAU’s vote to cement their preferred definition, but should rather use and teach the definition they find useful. In parallel, they should not begrudge other scientists’ criteria for what makes a definition useful to them. Just as in the example about the use of the term metal, each user community should use planet definitions useful to them without deferring to a central voting authority. And, just as other definitions arise organically, the definition of planet may now be considered organic, drawing to a close this public hand-wringing debate and thawing hearts that had frozen toward the planet Pluto.

Kirby D. Runyon is a postdoctoral planetary geologist at the Johns Hopkins Applied Physics Lab specializing in image analysis to understand the evolution of planetary landscapes.

S. Alan Sternis a planetary scientist who primarily studies the outer solar system. He is also the principal investigator on NASA’s New Horizons mission and formerly the associate administrator for NASA’s Science Mission Directorate.